A grain-oriented electrical steel sheet has been mainly used as a core material of electric appliances such as transformers, and must have excellent magnetic properties such as excitation characteristics, iron loss characteristics, and so forth. A magnetic flux density B in a magnetic field of 800 A/m (hereinafter called "B.sub.8 " in the present invention) is ordinarily used as the numerical value representing the excitation characteristics, while W.sub.17/50 is used as a typical numerical value representing the iron loss characteristics.
The magnetic flux density is one of the very important factors that govern the iron loss characteristics. Generally speaking, the higher the magnetic flux density, the better the iron loss. When the magnetic flux density becomes excessively high, however, secondary recrystallization grains become coarse, so that an abnormal eddy current loss becomes increase and the core loss may deteriorate. In other words, the secondary recrystallization grains must be appropriately controlled.
The iron loss comprises a hysteresis loss and an eddy current loss. The former is associated with purity, internal strain, etc, besides the crystal orientation of a steel sheet and the latter is associated with an electric resistance, a sheet thickness, etc, of the steel sheet.
The iron loss can be reduced by improving the purity and removing the internal strain as much as possible, as is well known in the art.
The iron loss can be reduced also by improving the electric resistance and reducing the sheet thickness. One of the methods of improving the electric resistance increases the Si content, for example, but this method has a limit because the production process or the workability of the product deteriorate when the Si content is increased.
Similarly, because a reduction in the sheet thickness results in the drop of productivity, an increase in the production cost will occur. Therefore, there is also a limit to the reduction of the sheet thickness.
A grain-oriented electrical steel sheet can be obtained by causing secondary recrystallization in finish annealing so as to develop a so-called "Goss texture" having {110} in the direction of the sheet plane and &lt;001&gt; in a rolling direction.
Typical production process of the grain-oriented electrical steel sheet are described in U.S. Pat. No. 1,965,559 owned by N. P. Goss, U.S. Pat. No. 2,533,351 owned by V. W. Carpenter and U.S. Pat. No. 2,599,340 owned by M. F. Littmann et al.
These production processes features that MnS is used as a principal inhibitor so as to cause the secondary recrystallization of the Goss texture at a high temperature during finish annealing, a slab is heated at a high temperature of not lower than 1,800.degree. F. so as to cause solid solution of MnS and cold rolling and annealing inclusive of intermediate annealing are carried out a plurality of times after hot rolling and before high temperature finish annealing. From the aspect of the magnetic properties, this grain-oriented electrical steel sheet satisfies the relationships of B.sub.10 =1.80 T and W.sub.10/60 =0.45 W/lb (2.37 W/kg in terms of W.sub.17/50).
As described above, the iron loss characteristics of the grain-oriented electrical steel sheet results from various factors. The method of producing the grain-oriented electrical steel sheet requires a longer production process and is more complicated than production methods of other steel products. Therefore, in order to obtain stable quality, a greater number of control items exist and this problem is a great burden to operating engineers. Needless to say, this problem greatly affects the production yield.
On the other hand, grain-oriented electrical steel sheets includes two types of the steel sheets, i.e. a high flux density grain-oriented electrical steel sheet having B.sub.8 (T) of at least 1.88 (JIS standard) and a CGO (Commercial Grain Oriented Silicon Steel) having a flux density of not higher than 1.88. The former mainly uses AlN, (Al.cndot.Si)N, Sb, MnSe, MnS, etc, as the inhibitor whereas the latter mainly uses MnS as the inhibitor. The producing methods vary also depending on the types of the products described above. Namely, the former includes a single (or one stage) cold rolling method and a double cold rolling method while the latter includes a second stage cold rolling method. In other words, there is hardly the case where the grain-oriented electrical steel sheet of the CGO grade is produced by the single cold rolling method, and the development of the grain-oriented electrical steel sheet of the CGO grade which can be produced by a shorter process and at a lower cost of production has been earnestly desired.